A wide variety of optical modulators have been developed in a continuing evolution. A number of active optical materials, for example, magneto-optical, electro-optical, ferroelectric, semiconductor, etc. are used with varying degrees of success. However, all compromise some aspects of their performance when modulation of large aperture, wide angle incidence signals is attempted. The magneto-optic modulators, generally speaking, tend to be operated only in a pulsed mode, they are expensive and have been found to require highly sophisticated driving electronics. Ferroelectric materials, even when interdigital electrodes apply the electric field, require a pair of polarizers that necessitate higher operating voltages and the consequent possibility of arcing between adjacent electrodes. Semiconductor and acousto-optical materials inherently possess limitations regarding their operational spectrums, usually the infrared spectrum with a material such as GaAs and Ge, and generally speaking, acousto-optic materials require high driving input levels which have a tendency to destroy or crack their substrate materials.
The conventional electro-optical modulators have not addressed themselves to maintaining beam divergence qualities of 10 to 100 microradians nor did they attempt to modulate large angles of incident energy up to plus and minus 45 degrees. Many of the old techniques primarily used large length-to-diameter ratios (long and thin crystals) and an electro-optic effect for modulation. An obvious shortcoming of this approach was the creation of a small acceptance angle and a fragile structure with a small cross section that produced relatively large effects on beam divergence greater than one milliradian Another disadvantage of the long narrow crystals was that they were lossy and not capable of transmitting high optical powers.
Balancing the constraints of large aperture, large acceptance angles of incidence, reduced power requirements and low cost of materials has tended to steer designers toward an electro-optical material such as a crystal of lithium niobate (LiNbO.sub.3) or lithium tantalate (LiTaO.sub.3) for visible radiation or gallium arsenide (GaAs) or cadmium telluride (CdTe) for infrared radiation.
An article dated July 19, 1973 and entitled "Low Voltage Optical Modulator Using Electro-Optically Induced Phase Gratings" by Toshio Motoki appeared in Volume 12, number 7 of Applied Optics. An electro-optic array modulator was fabricated from a number of LiNbO.sub.3 rods. The number of rods had to be precisely shaped and secured together in an alternating axis pattern so that a relatively low modulation potential could diffract impinging light passing through its longitudinal axes into many orders. A typical diffraction angle-far field pattern was produced; yet the long-rod modulator is limited in its acceptance angle of light and the cost and complexity of making a large aperture design appears to be prohibitive. In addition, this design might suffer a loss of transmissivity since the incident light waves must travel the length of the sandwiched rods and beam divergence could be excessive.
R. A. Meyer developed a multichannel phase modulator employing a crystal of lithium tantalate to perform one dimensional optical beam steering. His results are discussed at length in an article appearing in the Mar. 19, 1972 edition of Applied Optics, Volume 11, number 3 and entitled "Optical Beam Steering Using a Multichannel Lithium Tantalate Crystal". This modulator beam steers to 46 channels of light which traverses through a thick dimension extending along the X.sub.2 axis and along which are placed a number of parallel electrodes for altering the birefringence of the crystal. Masks aligned with the electrodes block a considerable portion of the light so that when considered with the crystal's thickness, transmissivity and acceptance angle are compromised.
A LiNbO.sub.3 electro-optic modulator was disclosed in the 20th of May, 1971 issue of Electronics Letters, volume 7, number 10. A beam compression lens directs a z polarized laser beam along the longitudinal y optic axis and a modulating voltage is fed to an interdigital electrode arrangement parallel to the y-z plane to effect a Pockel's -type linear modulation. Modulatable diffraction orders were observed with improved performance when a d.c. bias was added to the modulating voltage; however, wide aperture and acceptance angle in addition to a possible loss of transmissivity through the longitudinal y-axis may reduce the effectiveness of this design.
J. M. Hammer in his article entitled "Digital Electro-Optic Grating Deflector and Modulator" appearing in Applied Physics Letters, Volume 18, number 4 of February 1971 diffracted focussed light by electro-optic phase gratings. This technique was found to be particularly adaptable to thin-film light guides which require relatively low power for high speed operation. Differently dimensioned electrode columns were mounted on one side of a lithium niobate wafer and a ground was uniformly deposited on the other side to vary a periodic electric field that changed the refractive index. Light was diffracted into several orders along the Y direction. The angle of incidence and acceptance angle appear to be small since the differently dimensioned columns would change the diffracted orders as the angle of incidence was changed. Also, because light was transmitted through one of the larger dimensions of the crystal, losses may be unacceptable for lower energy signals. Furthermore, because of the differently dimensioned electrode columns, it might be quite expensive to fabricate.
One other approach that bears mention at this point is the electro-optical modulator disclosed in U.S. Pat. No. 3,958,862 issued to Marek Tadeusz Victor Scibor-Rylski on May 25, 1976. Incident light is transmitted in the general direction of the length of a lithium niobate crystal to come in close proximity or impinge upon a number of longitudinally disposed interdigital electrodes. The total internally reflected light is diffracted into different orders. Here again it appears that this modulator would not have an overly large aperture and would not lend itself to the reception, modulation, and transmission of light outside of a relatively narrow acceptance angle. The modulator also would tend to attenuate a light beam by transmitting the beam through an excessive amount of the electro-optical material.
Thus, there is a continuing need in the state-of-the-art for a wide acceptance angle-large apertured modulator of collimated optical signals operable in the tunable diffraction grating mode which is relatively inexpensive and capable of highly reliable operation.